426 CHINESE OPTICS LETTERS / Vol. 6, No. 6 / June 10, 2008

Optical system and lens design for

Blu-ray disc optical pick-up

Lihua Li (ooosssuuu), Longfa Pan (���999{{{), and Jianshe Ma (êêêïïï���)

Optical Memory National Engineering Research Center, Department of Precision Instrument and Mechanology,

Tsinghua University, Beijing 100084

Received September 21, 2007

During the design process of Blu-ray disc optical pick-up (BD OPU), the optical system and the collimatorlens design is especially important. This paper designs an optical system and some lenses for the BD OPU,including collimator lens, beam shaping lens for laser beam shaping, and cylinder lens for signal detecting.In this OPU, we use a triplet lens to collimate the laser beam. At the same time, we build a series ofassembly jigs to make sure that each lens can be put into the OPU basement properly. At last, we get thereading spot image and S-curve photo of OPU, which can be used to read Blu-ray disc (BD).

OCIS codes: 210.0210, 220.0220, 230.0230.

In the global market of the optical data storage, CD, CD-ROM, DVD-ROM, DVD-RW, DVD+RW and so on arewidely used. Although compared with CD, current DVDalready has considerable progress in storage density andread-write speed, the enhanced step is still restricted bythe red laser, which spurs the requirement of new gener-ation of blu-ray technology. The new technology uses theblue ray laser wave band to carry on the work, and thestorage capacity may increase nearly 5 times or more,causing a technological breakthrough of high density ofoptical data storage[1−3]. Table 1 introduces the formatfor Blu-ray disc (BD) with single layer.

In this paper, according to the format for BD withsingle layer, we build up a commercial optical systemfor the Blu-ray disc optical pick-up (BD OPU) and theray-path with optical elements is shown in Fig. 1.

As the selected laser diode (LD) does not have thefeedback function, we need to build a special optical partfor LD power control. Therefore, BD OPU optical sys-tem should be composed of the following three partialoptical ray paths: collimating, beam shaping and laserpower controlling. Table 2 gives the basic parameters of

Table 1. Format for BD with Single Layer

Capacity 23.3/25/27 GB (Single Layer)

Wavelength of the Laser 405 nm

NA of the Objective Lens 0.85

Data Transfer Rate 36 Mbps

Diameter of the Disc 120 mm

Thickness of the Disc 1.2 mm

Diameter of the Center Hole 15 mm

Recording Method Phase Change

Signal Modulation 1 − 7PP

Data Track Groove Recording

Addressing Method Wobble

Visual Data MPEG-2 Video

Audio Data AC3, MPEG-1, Layer2, Others

AV Multiplex Method MPEG-2 Transport Stream

SONY LD whose grade is SLD3233VF-53[4].From Table 2, the parallel and perpendicular radia-

tion angles can be obtained. These decide the numericalaperture (NA) of collimator lens.

In the collimation path of rays, we mainly designed thetriplet lens to avoid using asphere lens, because the costof asphere lens is much higher than triplet lens with thesame performance. In the asphere lens market, althoughthere are many types of ready-made products, speciallens for our LD is few and not suitable for this system.Therefore we designed the collimator lens according tothe theoretically requirements for the LD radiation angleand objective lens.

In BD OPU system, the basic requirement for objectivelens which is written in Blu-ray disc read-only format[5]

is that NA should be 0.85. The first objective lens with aNA of 0.8 was developed using a two-element lens to in-crease the fabrication tolerance in 1997[6]. Then in orderto correct chromatic aberration caused by mode hoppingand wavelength change of the laser diode, a three-elementobjective lens with NA of 0.85 at 405 nm was proposedin 2002[7]. Almost at the same time, the first singleobjective lens that has the NA of 0.85 was developedin Victor Company of Japan, Ltd. (JVC)[8]. Thenin 2003, JVC published the design principle for single

Fig. 1. Ray-path in BD OPU and optical components.

1671-7694/2008/060426-05 c© 2008 Chinese Optics Letters

June 10, 2008 / Vol. 6, No. 6 / CHINESE OPTICS LETTERS 427

Table 2. Parameters of the Sony Laser (SLD3233VF-53)(Electrical and Optical Characteristics T c = 25 ◦C)

Item Symbol Conditions Min. Type Max.

Threshold Current (mA) Ith CW − 35 65

Operating Current (mA) Iop CW, P0 = 65 mW − 80 110

Operating Voltage (V) Vop CW, P0 = 65 mW − 5.3 6.5

Wavelength∗ (nm) λp CW, P0 = 65 mW 400 405 410

Differential Efficiency (mW/mA) ηD CW, 5 − 65 mW 0.9 1.4 −

Radiation Angle Parallel ∗ (deg.) θ�CW, P0 = 65 mW

6.5 8.5 11

Perpendicular∗ (deg.) θ⊥ 18 21 24

Astigmatism (µm) As CW, P0 = 65 mW −6 − 0

Positional Accuracy Angle∗ (deg.) ∆φ�CW, P0 = 65 mW

− − ±2.0

∆φ⊥ − − ±2.5

Position (µm) ∆x, ∆y,∆z − − ±80

*Inspection standard.

objective lens with NA of 0.85[9]. Although at that timethis single lens is difficult to be produced, it makes lowercost possible. When the technology was developed, sin-gle lens cost less than the two- or three-element lens. In2004, Matsushita Electric Industrial Co., Ltd. developedthe compatible objective lens for BD and digital versa-tile disk (DVD)[10,11]. In 2005, Konica Minolta Co., Ltd.produced the plastic objective lenses for BD OPU[12].Till that time, the objective lens had been successfullydeveloped from research to product. In this paper, wechoose the objective lens produced by Konica MinoltaCo., Ltd. for BD OPU whose type is T963[13]. Thedetail data of the objective lens is supplied by PhilipsInvestment Co., Ltd. Shanghai R and D Centre[14].

In the proposed Blu-ray optical system, the objectivelens’s parameters are shown in Table 3. This objectivelens is a glass singlet (glass molded or glass replica). Itwill be used in a BD OPU, which is designed for singlelens DVD compatibility by means of a separate opticalcomponent[14].

The spot size is calculated according to the Airy disk[15]

R = 0.61λ/NA, (1)

where R is the spot radius and λ is the wavelength. How-ever, we usually use Eq. (2) to calculate the spot size:

R = 0.77λ/NA. (2)

We may obtain the Airy disk radius of the objective lens:

R = 369.6 nm. (3)

According to Table 2, we may obtain the laser hor-izontal divergence angle which is 8.5◦ and the verticaldivergence angle which is 21◦. So, we can calculate thefocal length of collimator lens:

fCL = D/2/tg(θ⊥/2) = 10.791, (4)

Table 3. Parameters of the Objective Lens

Focus Length (EFL) 2.35 ± 0.05 mm

Numerical Aperture (NA) 0.85 ± 0.01

Enter Pupil Aperture (D) 4.0 mm

where D is the enter pupil aperture. The NA of collima-tor lens is

NACL = tg(θ⊥/2) = tg(21/2) = 0.185. (5)

Therefore we may obtain the Airy disk radius RCL ofcollimator lens:

RCL = 0.77λ/NA = 1698.2 nm. (6)

According to the above theory and computation, we de-signed the triplet lens, and the parameters of the tripletlens are shown in Table 4.

Finally, we get the layout and the spot diagram of thecollimator lens as Figs. 2 and 3 show. From Fig. 3, whenfield angle is zero degree and in the ideal condition, thespot with 0.150 micron root-mean-square (RMS) radiusmay be obtained. When the field angle is 0.4◦, the spotis still very small. So this designed triplet lens is suitablefor the requirement of objective lens.

At the same time, we also evaluate the design resultsby Figs. 4 and 5. Through processing the collimator lens,we obtained the ellipse parallel light with 4-mm verticaldirection aperture and 1.6-mm horizontal direction aper-ture.

We designed the beam shaping ray path, and the prin-ciple is shown in Fig. 6. The main purpose is to shapethe ellipse parallel light with horizontal direction aper-ture D1 = 1.6 mm into round parallel light with verticaldirection aperture D2 = 4 mm diameter.

The shaping scale M can be calculated by

M = tg(θ⊥/2)/tg(θ�/2) = cos i2/ cos i1, (7)

M = tg(θ⊥/2)/tg(θ�/2) = 2.5/1. (8)

As we select the glass material K9, its refractive indexn2 is 1.53022, and according to the law of refraction (heren1 = 1.0):

n1 sin i1 = n2 sin i2. (9)

428 CHINESE OPTICS LETTERS / Vol. 6, No. 6 / June 10, 2008

Table 4. Parameters of the Triplet Lens

Surf: Type Radius Thickness Glass Semi-Diameter

OBJ Standard Infinity Infinity Infinity

ST0 Standard 9.410000 1.660000 BAK7 2.001503

2 Standard −7.780000 0.600000 ZF6 1.910298

3 Standard −22.446000 0.500000 1.883660

4 Standard 5.850000 0.900000 BAK7 1.770308

5 Standard 7.164000 7.955139 V 1.608767

6 Standard Infinity 0.250000 BAK7 0.194649

7 Standard Infinity 0.500000 0.166244

IMA Standard Infinity 0.075271

Fig. 2. 3D layout of collimator lens.

Fig. 3. Spot diagram of collimator lens.

Fig. 4. Optical path difference.

Fig. 5. Polychromatic diffraction MTF.

Associating Eqs. (7)—(9), we got the incidence angleand the beam shaper angle:

i1 = 58.231◦, i2 = 33.7647◦. (10)

In order to reduce the cost and optical elements manu-facture time, we used the fine production-cylindrical lensto make astigmatic aberration for focusing servo method.

As shown in Fig. 7, a cylindrical lens is applied in con-vergence ray path, so in order to match the cylindricallens, a convergent lens is set in front of the cylindri-cal lens. Detail ray path is shown in Fig. 8, and spotdiagram through focus on photonic detector integratedcircuit (PDIC) as shown in Fig. 9.

Based on the design, we put the LD in the holder andused the planar adjuster to adjust the holder, to makesure the direction of the ellipse beam spot straight, thenthe gum water was adopted to fix the holder. Then thecollimator and u-meter were used to adjust the under-side and side of OPU platform parallel the path; We put

Fig. 6. Shaping path.

Fig. 7. Cylindrical lens application ray-path.

Fig. 8. Detail ray-path.

June 10, 2008 / Vol. 6, No. 6 / CHINESE OPTICS LETTERS 429

Fig. 9. Spot diagram through focus on PDIC.

the collimator lens in the hole of the holder, and thenwe could see a beam spot in this receiver. The distancebetween LD and collimator lens was adjusted until thebeam spot did not change when we moved the receiver,which meant we got the parallel beam.

With the help of the angle test instrument, we orderlyplaced the beam shaping and polarization beam split-ter. We could also use the receiver to notarize the beamshaping and polarization beam splitter in the right place.

Then we used the collimator to adjust the mirror. Weadjusted the collimator to make the laser light plumbthe flat, and a red beam spot was observed in the centerof the receiver. Then a mirror was put after turning offthe collimator’s laser, and adjusting the mirror until wecan see a blue beam spot in the center of the receiver.Through adjusting the quarter wave plates, the beamspot was located in the center of the quarter wave plates.We used the collimator to make the objective lens plumbthe light until a perfect spot was observed.

Finally, we used a microscope to adjust the cylinderlens. We made an actuator which can hold the objec-tive lens to move up and down, so that we could seethe elliptical beam spot to vary to a circular beam spot,and then to an elliptical beam spot again. We revolvedthe cylinder lens, and then we can get this elliptic beamspot whose direction is about 45◦. To stabilize the laserpower, photonic detector (PD) board was mounted onthe OPU platform and automatic power control (APC)circuits worked properly. We used the planar adjusterto adjust the place of PDIC board until we can get agood S-curve and read-out frequency (RF) signal whilesearching.

After the assemble process, we got a good spot andS-curve, which shows that this system can be used toread Blu-ray disc. Figure 10, are the photos of the spotcoming through the objective lens and cylindrical lensand S-curve photo.

Basically, whether an optical system is good for OPUto read out the signal is judged by:

1) optical parameters including aberrations, spot sizeand power distribution.

2) focus error signal before closed loop (S-curve).In Fig. 10(a), the spot size measured by a simple spot

analyzer[16,17] is about 270 nm (1/2 of DVD spot size).All the aberrations, especially for coma, were reducedby analyzer at real time. Then in Fig. 10(b), the spotshape on PDIC is consistent with design result at defo-cusing time. In Fig. 10(c), the S-curve can be used forclosed loop as it is symmetric and clear enough. At thesame time, we also can see the small pinnacle before theS-curve and it is caused by reflection light of cover glass,and this can be disposed by other servo circuit.

Fig. 10. Spot photos of (a) objective lens, (b) cylindrical lens,and (c) S-curve photo.

Fig. 11. Photo of BD OPU.

In this paper, we designed an optical system includingcollimator lens, beam shaping lens and cylindrical lensfor BD OPU as shown in Fig. 11. A BD spot by CCDwas obtained and compared with the DVD spot. The BDspot is nearly half of the DVD spot. From the S-curve,the results show that it can focus and get the signal ondisc.

This work was supported by the project cooperated byOptical Memory National Engineering Research Center,Tsinghua University and Philips Investment Co., Ltd.Shanghai R and D Centre. Many thanks for the helpof Henk Goossens, Deer Yi and Bei Wang from PhilipsInvestment Co., Ltd. Shanghai R and D Centre; Toshi-hiko Ohtomo, Toru Suzuki and Hisashi Arakaki fromDigital Stream Corporation. H. Li’s e-mail address islilihua2002@126.com.

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